1. Field of the Invention
The invention relates to a suction-controlled ring gear pump, in particular oil and/or hydraulic pump for motor vehicles and/or transmissions.
2. Description of the Prior Art
The drive of the pump is usually by the shaft carrying the pinion. Such pumps are used for example for supplying hydraulic systems. Such pumps are known from DE 39 33 978 C2 of Applicants. The latter corresponds to U.S. patent application Ser. No. 593,714, now U.S. Pat. No. 5,096,397, and Japanese patent application 3-175182.
Motor vehicle engines and transmissions in particular are operated in a wide speed range. The speed of rotation limit values may be in the relationship 10:1 or more.
In contrast, the nominal displacement of the lubricating pump of a motor vehicle engine, which with automatic transmissions must additionally perform the function of pressure supply of the hydraulic switching elements and the converter filling against cavitation, both in the case of the engine and in the case of the transmission, is substantially proportional to the speed of rotation only in the lower part of the operating range. In the upper speed range the oil requirement increases far less than the speed of the engine. Consequently, a drive-regulated lubricating or hydraulic pump or a pump with a displacement adjustable depending upon the speed is required.
The practical characteristic of the displacement with respect to the speed depends on a multitude of parameters, such as delivery pressure, oil viscosity, flow resistance in the suction and pressure conduit, configuration of the teeth of the gears, width of the gears and design of the pump. For approximate adaptation of the displacement curve to the requirement curve, for example of an internal-combustion engine, suction, regulation has been developed. By using correspondingly narrow suction conduits or by an orifice or in regulatable manner by a suction slide valve, the flow resistances in the suction pipe may be fixed so that a certain adaptation of the useful displacement of a gear pump to the requirement curve of the consumption is achieved. This is known for example from DE 36 27 414 A1. According to the latter three parallel suction conduits are provided, two of which have valves controlled in dependence upon operating parameters of the engine whilst a rigid orifice is disposed in the third suction conduit. DE 36 27 414 A1 describes by the way a ring pump of different type with filling piece. With this pump it is hardly possible to achieve satisfactory sealing of the cells with respect to each other where it is important, i.e. between filling piece and engagement point.
A disadvantage of this suction control is the cavitation which occurs. The latter leads to implosions of the gaseous constituents of the cell content so that undesired noises result, and, even worse, destructions of the cell walls.
To avoid these implosions, in the pressure region of the pump the cell content is given time by gradual reduction of the cells to increase the static pressure so that at the instant at which the cell enters into communication with the outlet passage, at least theoretically, no implosions of gas bubbles can occur because by this gradual reduction of the cell volume the bubbles have already condensed to liquid again or have dissolved in the liquid. The "slow" compression of the vapour and air spaces can be ensured constructionally in that on the displacement side of the pump the cells are connected to the displacement press,ire chamber initially only via check valves so that when a cell is not completely filled with fluid the displacement pressure cannot be effective therein.
If however the cells are already completely filled with fluid on the suction side, as is the case in the low speed range, the higher squeezing pressure in the cell opens the check valve in the direction of the pressure delivery space so that the displaced oil can flow into the pressure space with only slightly increased cell pressure compared with the delivery pressure corresponding to the opening pressure of the check valve and the flow resistance thereof. Such a construction is also known from DE 30 05 657 C2. In the latter axial bores leading to the outlet passage extend over the entire pressure half of the pump in the housing and contain spaced from the gear chamber check valves which open only when the pressure of the cell lying in front of the respective bore exceeds the pressure in the outlet passage. Accordingly, like the pump according to DE 36 27 414 A1, this pump has a large axial extent. The spring valves used may vibrate and break. Also, the irregular connection of the displacement cells to the outlet passage is disadvantageous. Finally, the pressure distribution is also disadvantageous as regards the use of cavitation-induced implosions.
These disadvantages are avoided in the pump of the type according to the application as set forth in DE 39 33 978 C2. In is short and has a small diameter, a favourable pressure profile in the pressure range, can be installed in existing constructions subsequently to replace the lubricating pump, is reliable in operation and has a simple construction. The housing is simply constructed and has only a small axial extent. Since each diminishing displacement cell can pass operating fluid only into the displacement cell in front, the pressure in each displacement cell is increased only gradually in the diminishing range until the pressure has reached the value at the outlet opening. A particular advantage here is that due to the passages with the ball valves a quite considerable flow resistance exists between the adjacent displacement cells. Preferably, the mouths of the inlet and outlet passages are arranged in the end walls of the gear chamber as so-called inlet and outlet kidneys. This permits very large influx and efflux cross-sections into and out of the displacement cells. The overflow passages may preferably be arranged in the teeth of the gears. The check valves may be formed as ball valves, the ball tending in each case to press against the valve seat due to the centrifugal force of the rotational movement of the gear containing the valves.
If in a suction-controlled ring gear pump the throttle in the inlet passage is controlled in such a manner that with increased fluid requirement the throttle cross-section is enlarged, for example by opening a throttle flap in a by-pass passage (DE 3 627 414 A1) (such a situation arises for example with the oil pump of a motor vehicle engine when an exhaust gas turbocharger is connected) in order to cause the displacement characteristic to become horizontal only at higher speed, the filling degree of the displacement cells in the suction range is increased with the opening of the throttle.
This results on the outlet side of the pump in an increased flow of the fluid through the overflow passages because the increased amount of fluid must be expelled. This leads to an impairment of the efficiency and to a reduction of the desired increase in the pump throughput.